The substitute question might be a bit tricky, as it is hard to access any rule books. You might need to find and study match reports to deduct that information.

Regarding your question about the foreigner quota, the 1980s were indeed very intense. Based on our archival research, the HKFA allowed 7 foreign players, however reduced the number to 6 for 1983/84, to three for 1984/85, and to two for 1985/86, before eventually banning all foreign players from 1986 onwards. This was a controversial decision, which was widely discussed in the news, and it is allegedly also one of the main reasons for Seiko to pull out of the league. Apparently, there have also been a few exemptions (e.g. for foreign players having lived in HK for 3 years – as well as a relaxed quota for the HKFC and the Police Forces team). It seems the ban was lifted in 1989/90, when teams were allowed to field 2 foreign players again.

We extracted this information from the analogue South China Morning Post archive, so the information here might not be 100% accurate, as it is just based on occasional news stories (and not the actual rule book of HKFA at that time).